Balanced circuit arrangement and method for linearizing such an arrangement

ABSTRACT

The present invention relates to a balanced circuit arrangement and methods for linearizing and calibrating such a circuit arrangement, wherein linearization is obtained by introducing a load imbalance between the output branches of the balanced circuit arrangement. Thus, a controllable extraneous imbalance is created between the output loads of the balanced circuit arrangement to thereby obtain a linearization by means of even-order non-linearity.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation of Application Ser. No. 10/474,848 filed Oct. 15,2003, which is a 371 of PCT/EP01/04414 filed Apr. 18, 2001. Thedisclosure of the prior application(s) is hereby incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to a balanced circuit arrangement such asa balanced or double-balanced amplifier or multiplier and to a methodfor linearizing such an arrangement e.g. in order to attenuate spurioussignals and envelope distortions in radio receivers and transmitters.

BACKGROUND OF THE INVENTION

In radio reception, the linearity of the receiver is an essentialrequirement. The linearity performance of a receiver, in general, isusually dominated by the downconversion mixer circuits. Such mixercircuits are used to translate or convert received high frequencysignals down to a lower or intermediate frequency (IF). This conversionis achieved by mixing the received signals with a locally generatedoscillator signal. By choosing the local oscillator signal to be aconstant amount away from a selected or wanted signal in a firstfrequency band, the selected or wanted signal always appears at the samefrequency in the intermediate frequency band. Thereby, selection of theselected or wanted signal may be facilitated by a fixed-tuned IF filter.

In homodyne or direct conversion receivers, the chosen intermediatefrequency band is DC or zero frequency. The local oscillator then has azero frequency separation from the selected or wanted signal. Anymodulation on the selected or wanted signal that causes spectralcomponents both above and below the nominal signal frequency becomesfolded at the mixer output, as a component below the signal frequency orabove the signal frequency will appear at the intermediate frequencyabove the nominal of zero. To allow for resolution of such foldedcomponents, two mixers are provided in a direct conversion receiverusing local oscillator signals that are phase offset by 90 degrees. Thecomponents above and below the nominal signal frequency then appearfolded as a sum signal at one mixer output and a difference signal atthe other mixer output where they may be separated if desired. Suchdirect conversion receiver operations are described in more detail indocument U.S. Pat. No. 5,241,702.

However, due to non-linearities, spurious responses will be generated inthe direct conversion receiver, the worst being modulation-frequencyinterference at the receiver's mixer output caused by a strongamplitude-modulated signal of another transceiver. This will appear evenif the frequency of the interfering signal considerably deviates fromthe receiving frequency. These interferences are mainly caused by thesecond-order distortion component which contains a variable-level DCcomponent proportional to the amplitude of the interference-causingsignal. The variable amplitude signal produces at the mixer output asignal which comprises a variable DC component and the frequency ofwhich is identical with the variation of the amplitude. The spuriousfrequencies may corrupt the radio reception by blocking the followingsignal processing stages or deteriorating the detection of the desiredsignal which is overwhelmed by distortion.

The spurious frequencies can be categorized to exist due to the odd- andeven-order non-linearities. The even-order mixing results are suppressedby using balanced or double-balanced mixer topologies. Ideally, theeven-order spurious frequencies are cancelled in balanced anddouble-balanced constructions. However, in practice, the canceling isimperfect. The reason is the imperfect balance due to the mismatch ofrespective components in the differential branches, i.e. manufacturingtolerances.

In radio receivers utilizing a direct conversion architecture or asignificantly low IF, the spurious signals cannot be removed byselecting an optimal IF. Due to the existence of the even-orderdistortions and imbalance in the circuitry, a variable DC componentproportional to the signal level and amplitude modulation depth of theinterfering signal occurs. Moreover, envelope distortions are detected,the amplitude of which is also proportional to the amplitude modulationdepth of the interfering signal, and the frequency of which equals tothe variation of the amplitude. Thus, not only a DC offset but also alow frequency disturbance may be generated to corrupt the desiredreception band. This is a particular concern in down-conversion mixersof direct conversion receivers.

Several solutions for reducing distortions in radio receivers with lowIF have been proposed. Document U.S. Pat. No. 5,749,051 suggestscompensating unwanted terms caused by second-order intermodulation byfeeding instantaneous power measurements to a signal processing unitalong with the complex baseband signals. The signal processing unit thendetermines a complex compensation coefficient by correlating the powersignal with the complex baseband signals. The complex compensationco-efficient is then employed to subtract a weighted amount of the powersignal from the complex baseband signal in order to cancel the unwantedsecond-order intermodulation distortion terms. Furthermore, document EP0 951 138 discloses a method for attenuating spurious signals in mixercircuits by setting variable-level bias voltages and/or currents totransistors in the mixer circuits. Additionally, document GB 2 346 777suggests switching a DC offset correction in or out of the circuitryaccording to the received signal strength or signal-to-noise ratio.

Furthermore, the use of a dynamic matching procedure is described by E.Bautista et al., “Improved Mixer IIP2 through Dynamic Matching”, in theDigest of ISSCC 2000, pp. 376-377. According to this procedure, anyundesirable second-order intermodulation distortion product generated inthe mixer circuit is modulated to a frequency where it can easily befiltered off. This can be achieved by applying a periodic signal toinput switches of the mixer circuit in order to modulate the receivedinput signal. If the periodic signal is replaced by a pseudo-randomsignal, the undesirable second-order intermodulation distortion productscan be spread over a wide range of frequencies to achieve a desiredsecond-order input intercept point (IIP2) performance.

The second-order distortion phenomena itself and its undesired products,i.e. DC offset and envelope distortions, have not been thoroughlyinvestigated so far. Due to lack of proper analysis of this topic, mostof the solutions have been focused on the removal of the DC offset.However, even if the DC offset at the output of the mixer circuit isreduced to zero, the circuit may still be in an imbalanced condition,due to the fact that the envelope distortion itself causes a DC termwhich is related to other DC offsets in a complex manner.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a balancedcircuit arrangement and method for linearizing such an arrangement, bymeans of which spurious signals and envelope distortions in radioreceivers and transmitters can be reduced.

This object is achieved by a balanced circuit arrangement comprising:

-   first transistor means coupled via first load means to a power    source;-   second transistor means coupled via second load means to said power    source; and-   linearity control means for linearizing the balanced circuit    arrangement by adjusting the first and second load means into a load    imbalance.

Furthermore, the above object is achieved by a method for linearizing abalanced circuit arrangement, comprising the steps of:

-   providing a controllable load means in at least one output branch of    the balanced circuit arrangement; and adjusting the load of the    controllable load means to obtain a linearizing load imbalance in    the output branches of the balanced circuit arrangement.

Additionally, the above object is achieved by a method for calibrating abalanced circuit arrangement, comprising the steps of:

-   measuring a DC output level or a lowpass filtered signal level of    said balanced circuit arrangement when no input signal is applied or    when input signal changes have only a weak effect on the measured    signal;-   supplying a test signal to the input of said balanced circuit    arrangement and measuring the DC output level;-   determining a difference between said measured DC levels; and-   adjusting a load means in at least one output branch of said    balanced circuit arrangement until said determined difference is    minimized.

The input signal can be either connected or disconnected from the mixerinput. In the former case, it should be assumed that the average levelof the AM distortion in the input signal varies only a little during thecalibration.

Accordingly, envelope distortions and even-order spurious signals arecontrolled by controlling or changing DC offset balance errors in thebalanced circuit arrangement. The balance errors occur due to themismatch of components, i.e. component manufacturing tolerances.Thereby, the balanced circuit arrangement can be linearized in terms ofeven-order non-linearity by introducing a controlled imbalance in theload of the balanced circuit arrangement. In particular, the inventioncan be applied to any balanced circuit arrangement in radio receiversand/or transmitters requiring even-order linearity, such as in mobilecommunications devices.

Thus, the second-order input intercept point (IIP2) can be maximized byadjusting the loads of balanced circuit arrangement into a slightimbalance. This can be achieved by creating a controllable extraneousimbalance between the output loads of the balanced circuit arrangement.According to the measurements and simulations, the IIP2 performance ofthe balanced circuit arrangement can be enhanced up to 30 dB. Theimbalance in the DC voltage or lowpass filtered and averaged signal,generated between the output branches can then be cancelled at a laterstage after the signals causing envelope or other distortions have beenfiltered out. The effect on other significant performance parameters isnegligible due to the fact that the required artificial mismatch isrelatively small compared to the absolute values of the load components.

According to an advantageous development, the linearity control meansmay be arranged to perform the adjustment by selectively switching loadelements to at least one of the first and second load means. Preferably,the load elements may be weighted load elements. Thereby, the amount ofload imbalance may easily be controlled by a switching control functionbased on corresponding control inputs. In case of a use of weighted loadelements, a control based on binary control words can be implemented.Thus, the linearity control means may comprise at least one inputterminal for inputting a control signal.

The balanced circuit arrangement may be a single- or double-balancedcircuit arrangement. In general, the present invention is applicable toany mixer circuit comprising a balanced circuit arrangement, e.g. aGilbert-cell multiplier. Furthermore, the present invention may beapplied to any modulator and/or demodulator circuit, such as an IQmodulator and/or demodulator, or to any receiver and/or transmittercircuit comprising e.g. a direct conversion receiver, in which abalanced circuit arrangement can be used.

According to another advantageous development, the adjusting step may beperformed by selectively switching load elements of the controllableload means.

Furthermore, the adjusting step of the calibrating method may be aniterative step and the difference may be monitored by an analog or by adigital signal processing routine.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail based on preferred embodiments with reference to the accompanyingdrawings, in which:

FIG. 1 shows a schematic block diagram of a receiver in which thepresent invention can be applied;

FIG. 2 shows a circuit diagram of a Gilbert-cell multiplier as anexample for a balanced circuit arrangement in which the presentinvention can be applied;

FIG. 3 shows a schematic block diagram of a calibration techniqueaccording to a preferred embodiment of the present invention;

FIG. 4 shows a flow diagram of a calibration procedure according to thepreferred embodiment of the present invention;

FIG. 5 shows a circuit diagram of a load controller which can be used inthe preferred embodiment;

FIG. 6 shows a double-balanced mixer as a preferred embodiment of adouble-balanced circuit arrangement;

FIG. 7 shows a circuit diagram of a single-balanced mixer as a preferredembodiment of a single-balanced circuit arrangement;

FIG. 8 shows simulated IIP2 performance characteristics for differentfixed imbalances between the output branches; and

FIG. 9 shows a measured trimming performance of an implementedintegrated direct conversion receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described on the basis of asingle-balanced and double-balanced mixer circuit, as an examples for abalanced circuit arrangement according to the present invention, whichmay be used in a direct conversion receiver for WCDMA (Wideband CodeDivision Multiple Access) base station applications in 3rd generationmobile communication networks.

FIG. 1 shows a block diagram of those parts of the receiver which areessential to the present invention. The receiver comprises an antenna 1from which a signal is received and filtered by a bandpass filter 2 andamplified by an amplifier 3. The amplified signal at the radio receptionfrequency is then mixed to the (zero) intermediate frequency in abalanced mixer 6 according to the preferred embodiments of the presentinvention. A local oscillator 5 feeds a predetermined frequency to themixer 6, such that the radio reception frequency is mixed with thedetermined frequency of the local oscillator 5 to obtained the (zero)intermediate frequency. The linearity of the mixer 6 is controlled by acontroller 4 by adjusting the load imbalance between the output branchesof the mixer 6.

FIG. 2 shows a known Gilbert-cell used as a balanced circuit arrangementin amplifiers, multipliers and mixers, such as the mixer 6 of FIG. 1. Inthe Gilbert-cell, two input voltages V_(X) and V_(Y) are multiplied intoone output voltage V_(OUT), i.e. the voltage difference at the outputsV_(OUT)+ and V_(OUT)− corresponds to the product or multiplication ofthe input voltages. The first input voltage V_(X) is applied between thebase terminals of transistors Q1, Q2 and Q3, Q4, respectively. Thesecond input voltage V_(Y) is amplified by transistors Q5 and Q6. Thetransistors Q5 and Q6 are connected at their emitter terminals to atransistor Q7 which is controlled by a bias voltage and coupled to knownpotential. The output branches of the circuit are coupled to a positiveoperating voltage through load impedances Z_(La) and Z_(Lb). Thetransistors Q1 and Q4 amplify the first input voltage V_(X) in a firstpolarity direction, and the transistors Q2 and Q3 amplify the firstinput voltage V_(X) in a second polarity direction opposite to the firstpolarity direction. The amplified voltages are coupled andcross-coupled, respectively, to the outputs V_(OUT)+ and V_(OUT)−.

According to the present invention, the load values of the loadimpedances Z_(La) and/or Z_(Lb) are controlled by the controller 4 so asto introduced a load imbalance required to maximize the IIP2 performanceof the multiplier or mixer circuit. It is noted that the load impedancesZ_(La) and Z_(Lb) may be any load means or circuitry which provides amechanism for adjusting the effective load value introduced into therespective output branch. Thereby, the IIP2 performance of the mixercircuit can be improved, since the DC and low-frequency effects of theenvelope distortions can be compensated by the introduced loadimbalance.

In general, any transceiver, receiver or transmitter circuit can becalibrated by properly adjusting the load imbalance of an includedbalanced circuit arrangement. A corresponding calibration method isdescribed in the following.

FIG. 3 shows an arrangement by means of which the calibration techniquecan be implemented. In particular, FIG. 3 shows a direct conversionreceiver arrangement comprising a low noise amplifier (LNA) 10 which isconnected to an antenna (not shown) for receiving radio frequencysignals. A received radio frequency signal is supplied to a bandpassfilter 20 (optional) via a switching element or switch 11. The“bandpass” filter can be a simple ac-coupling device as well. Inputsignal can be also switched on and off by biasing of the preceding LNA10, instead of switch 11. After the received signal has been filtered inthe bandpass filter 20, it is supplied to the mixer 6, where it is mixedwith a receiving local oscillator (Rx LO) 5 of a predetermined frequencyrelated to the radio reception frequency. The signal converted to the IFor zero frequency is supplied to a following receiver circuitry 7,comprising a test monitoring circuitry 12, where the signal is detectedat one of the test nodes 13 and distortions are extracted to obtain ameasurement for the second-order non-linearity performance (i.e. IIP2performance) of the receiving or mixer circuitry. A control signalcorresponding to the IIP2 performance is supplied by the test monitoringcircuitry 12 to a digital signal processor (DSP) 8 which evaluates theperformance and supplies a corresponding control signal or controlinformation to the controller 4 arranged to control the load imbalancein the output branches of the mixer 6. The procedure can be doneindependently in both in-phase (I) and quadrature (Q) branches, e.g., ina direct conversion receiver. However, only one signal and one controlpath is drawn in FIG. 3.

To obtain a reference or test signal, a transmission local oscillator(Tx LO) 50 is connected via an amplifier circuit 9 and a secondswitching element 14 to the input of the mixer 6. Thus, either areceived radio signal or the generated test signal or both can beswitched to the bandpass filter 20. The test signal can be any locallyor externally generated signal.

FIG. 4 shows a flow diagram of a calibration method according to apre-rred embodiment. This calibration method is used to adjust or set inthe output branches of the mixer 6 by the controller 4 in order toobtain a maximum linearization of the mixer 6 by the means of even-orderlinearity. This calibration method can be used to calibrate the receivercircuit during a stand-by period or during relatively stable receptionconditions, i.e. when input signal changes have only a weak effect onthe measured signal.

After the start of the method in step S100, the switching element 11 isclosed to connect the LNA 10 while no input signal is supplied to themixer 6. In step S101, it may be considered or checked whether thereceiving local oscillator 5 is switched on or off, since this may leadto different measuring results. Then, the output DC or lowpass filteredsignal level of the mixer 6 is measured in step S102 by the testmonitoring circuitry 12 at the respective one of the test nodes 13 andthe measuring result is supplied to the DSP 8 where it may be stored. Instep S103, the second switching element 14 is closed to connect to theoutput terminal of the amplifier circuit 9 so as to supply a sample ofthe test signal, e.g. the signal from the transmission local oscillator50 in a receiver calibration, to the mixer 6, wherein the amplifiercircuit 9 is arranged to provide an interface equivalent to that of theLNA 10. Alternatively, an attenuated test signal can be connecteddirectly-to the input of the LNA 10, while the second switching element14 remains open or can be dispensed with. Due to the second-ordernon-linearity and imbalance in the mixer 6, a DC error voltage isgenerated at the output of the mixer 6. This DC error is proportional tothe amplitude of the even-order spurious signal, and is measured and maybe stored in the DSP 8.

Based on the measured DC outputs, the DSP 8 provides a control to thecontroller 4 so as to adjust the load imbalance and thereby minimize theincrement or increase in the DC voltage or in the lowpass filteredoutput signal at the output of the mixer 6 due to the DC error. Thus,the receiver circuit can be linearized by this DC level set control.According to FIG. 4, the calibration process may be an iterative processand the DC error can be monitored by the routines of the DSP 8 e.g. viaA/D converters (not shown). After each iteration, a check is performedin step 105 as to whether a satisfactory result has been achieved, i.e.whether the circuit has been linearized to a sufficient extent. If not,step 104 is repeated. When a satisfactory result is determined in step105, the flow proceeds to step 106 where the user or a system isinformed of the system ready state.

Thus, an automatically controlled calibration of receiver, transmitteror transceiver circuits can be provided.

FIG. 5 shows a controllable adjustment block or circuitry forcontrolling the load value of the load elements Z_(La). The sameadjustment circuitry may be provided at the other load element Z_(Lb).According to FIG. 5, the load adjustment or control arrangementcomprises a plurality of load elements Z_(L1) to Z_(Ln) which can beswitched to be connected in parallel to the load element Z_(La) so as todecrease or increase the total load based on binary control signalsprovided to the control terminals ctrl1 to ctrlN. As an example, alogical signal “1” may be used to close the respective switch and toconnect the respective load element in parallel to the load elementZ_(La). Thus, the load value generated by the load adjustment circuitrycorresponds to the binary digits of the binary control word applied tothe control terminals ctrl1 to ctrln.

The load elements Z_(L1) to Z_(Ln) may be arranged to provide weightedloads i.e. their load values may be generated by multiplying a basicload value with an integer number. As an example, the load value of anindividual load element may be generated based on the followingequation:Z _(Li)=2^((i-1)) ·k·Z _(L)wherein Z_(L) denotes a basic load value or unit load value, and kdenotes an integer (1 . . . n) selected according to the desired tuningrange. Due to the fact that the above equation corresponds to theweighting of a dual number system, a direct relation between the binarycontrol word applied to the control terminals ctrl1 to ctrlN and thegenerated load value can be obtained, while the control terminal ctrlcorresponds to the most significant bit (MSB) and the control terminalctrlN corresponds to the least significant bit (LSB). The adjustmentcircuitry comprises weighted fingers of load elements connected inparallel to a basic or original load. Each finger can be selected usingthe respective switch. Thus, in case resistor loads are used, thecontrol terminal ctrlN connects the largest parallel resistor finger inparallel to the basic load Z_(L) to thereby obtain a minimum loadchange. The resolution of the load adjustment control can be selected toachieve a desirable range, e.g. the resolution may be 0.1%. Asdetermined on the basis of simulations and measurements, the totaltuning range should preferably cover a range of±5% of the basic ororiginal load value.

In complex receiver, transmitter or transceiver arrangements, eachmixer, modulator or other balanced circuit arrangement can be adjustedseparately. The calibration sequence may include counting of digitalcodes for adjusting the load imbalance, wherein measurements are swappeduntil the accepted level is reached.

FIG. 6 shows a double-balanced multiplier mixer circuit similar to theGilbert-cell shown in FIG. 2, wherein the bias adjusting transistor Q7has been removed. Nevertheless, the circuitry of FIG. 6 may as wellinclude the bias adjusting transistor Q7.

According to the embodiment of FIG. 6, both loads Z_(La) and Z_(Lb) canbe adjusted by respective load adjustment circuits C+41 and C−42respectively, connected in parallel to the respective load in therespective output branch. Thus, an individual or branch-selective loadadjustment can be performed by the controller 4. Both load adjustmentcircuits 41 and 42 may be arranged as shown in FIG. 5.

As can be gathered from FIG. 6, a local oscillator voltage V_(LO)generated by the receiving local oscillator 5 is applied between thebase terminals of the transistors Q1 and Q2 and between the baseterminals of transistors Q3 and Q4, while a radio reception frequencyvoltage VRF obtained from the LNA 10 is supplied between the baseterminals of the transistors Q5 and Q6. Thereby, the output voltageV_(OUT) between the collector terminals of the transistors Q1 and Q3 andthe transistors Q2 and Q4 corresponds to a multiplication of the localoscillator voltage V_(LO) and the radio reception frequency voltageV_(RF). The load value can be trimmed in both output branches to obtaina desired load imbalance.

FIG. 7 shows an embodiment of a single-balanced mixer or multipliercircuit, where the load of only one output branch can be adjusted by thecontroller 4. In particular, a load adjustment or control circuit C+41is connected in parallel to the load Z_(La) of one output branch. Thus,load trimming is performed in one output branch. In this case, the loadvalue of the other load Z_(Lb) should be selected according to thefollowing equation:Z _(Lb) =Z _(La)·(1−0.5 Δz),wherein Δz indicates the tuning range of the load adjustment circuit 41in percentages relative to the actual load. Thereby, an adjustment ofthe load imbalance is possible by the same amount in both directions.

It is noted that both single-balanced and double-balanced mixers can belinearized by trimming in either both output branches or one of thebranches

FIG. 8 shows characteristics of a simulated IIP2 performance of a mixercircuit versus the mismatch in the load values or resistances, achievedby the load adjustment circuits 41 and/or 42. The differentcharacteristics have been obtained for different fixed imbalances in themixer circuit. The behavior is similar and repeatable for bothsingle-balanced and double-balanced mixer circuits or other balancedcircuit arrangements. As can be gathered from FIG. 8, depending on theinitial fixed imbalances, which may be the result of componentmismatches, the performance peaks of the IIP2 performance are located atdifferent percentual load mismatch values. Thus, if the correspondingload mismatch value is introduced into the mixer circuit, an optimizedIIP2 performance can be achieved.

FIG. 9 shows a measured trimming performance of an implementedintegrated direct conversion receiver. According to FIG. 9, themeasuring results correspond to the simulated characteristics shown inFIG. 8. In particular, a sharp peak in the IIP2 performance of thedirect conversion receiver is obtained at a load mismatch ofapproximately 0.7%. Thus, the iterative calibration procedure will leadto a load adjustment to achieve this optimal load mismatch.

It is noted that the present invention is not restricted to the concretecircuit arrangements described in the preferred embodiments. The loadadjustment may be performed by any control means or elements which aresuitable to change or control load values in at least one of therespective output branches. In particular, the load adjustment may berealized by active elements such as bipolar or unipolar transistors,diodes or other semiconductor elements. Furthermore, the balancedcircuit arrangement may be provided in IQ modulators, multi-carrier ormulti-frequency receiver and/or transmitter systems. Furthermore, thecalibration signals used for adjusting the load adjustment circuit canbe obtained by using modulated carriers to allow for a greaterflexibility of calibration. The measuring of the DC level or the lowpassfiltered test signal may be performed solely by the DSP 8, such that thetest monitoring circuitry 12 can be dispensed with. The above preferredembodiments may thus vary within the scope of the attached claims.

1-19. (canceled)
 20. A radio frequency mixer circuit responsive to alocal oscillator signal and to an input radio frequency signal andfurther comprising a first output providing a first radio frequencymixer output signal and a second output providing a second radiofrequency mixer output signal, said mixer circuit further comprising afirst pair of transistors coupled to said first output and to a firstload and a second pair of transistors coupled to said second output andto a second load, said first load comprising a first plurality of inputcontrol signals to set a value of said first load, said second loadcomprising a second plurality of input control signals to set a value ofsaid second load.
 21. The radio frequency mixer circuit as in claim 20,where the first and second radio frequency output signals have oppositepolarities.
 22. The radio frequency mixer circuit as in claim 20, wherea value of at least one of said first load and said second load is setto induce a load imbalance condition and linearize at least oneeven-order non-linearity of the radio frequency mixer circuit.
 23. Theradio frequency mixer circuit as in claim 20, where a value of at leastone of said first load and said second load is set to control a level ofa DC error voltage output from the radio frequency mixer circuit. 24.The radio frequency mixer circuit as in claim 20, where a value of atleast one of said first load and said second load is set to increase avalue of a second-order input intercept point (IIP2).
 25. The radiofrequency mixer circuit as in claim 20, comprising a part of an in-phase(I) or a quadrature (Q) branch.
 26. The radio frequency mixer circuit asin claim 20, comprising a part of a direct conversion radio frequencyreceiver.
 27. The radio frequency mixer circuit as in claim 20,comprising a part of a receiver that is configured to operate in a WideBand Code Division Multiple Access (WCDMA) mobile communication network.28. A method, comprising: applying a local oscillator signal and aninput radio frequency signal to a radio frequency mixer and outputtingat a first output a first radio frequency mixer output signal and at asecond output a second radio frequency mixer output signal, where saidfirst output is coupled to a first load and said second output iscoupled to a second load; and applying a first plurality of inputcontrol signals to set a value of said first load and a second pluralityof input control signals to set a value of said second load.
 29. Themethod as in claim 28, where the first and second radio frequency outputsignals have opposite polarities.
 30. The method as in claim 28, wherethe value of at least one of said first load and said second load is setto induce a load imbalance condition and linearize at least oneeven-order non-linearity of the radio frequency mixer.
 31. The method asin claim 28, where the value of at least one of said first load and saidsecond load is set to control a level of a DC error voltage output fromthe radio frequency mixer.
 32. The method as in claim 28, where thevalue of at least one of said first load and said second load is set toincrease a value of a second-order input intercept point (IIP2).
 33. Themethod as in claim 28, performed in an in-phase (I) or a quadrature (Q)branch.
 34. The method as in claim 28, performed in a direct conversionradio frequency receiver.
 35. The method as in claim 28, performed in areceiver that is configured to operate in a Wide Band Code DivisionMultiple Access (WCDMA) mobile communication network.